2010 SKA Africa Bursary Conference Chalmers University of Technology Jian Yang, Associate Professor Chalmers University of Technology Sweden

2010 SKA Africa Bursary Conference Chalmers University of Technology Introduction New Method: the partial array method Optimization Procedure Result of Optimization Simulations and Measurements Conclusions

2010 SKA Africa Bursary Conference Chalmers University of Technology Two unique characteristics over decade bandwidth –Constant beam width; –Fixed phase center location dipoles Ground plane Phase center

2010 SKA Africa Bursary Conference Chalmers University of Technology Simple geometry, small volume –Can be located in cryostat; -10 dB reflection coefficient Low cross pol. level

2010 SKA Africa Bursary Conference Chalmers University of Technology Minimizing reflection coefficient is needed. Challenge: Eleven feed is very large at highest frequency. New method for global optimization scheme. Photo of the 2-13 GHz Eleven feed

2010 SKA Africa Bursary Conference Chalmers University of Technology If the log-periodic array is infinite, we have the frequency scaling on s-parameters as: idipole jdipole jdipole+n idipole+n

2010 SKA Africa Bursary Conference Chalmers University of Technology example: D1 D2 D3 D4 D5 D

2010 SKA Africa Bursary Conference Chalmers University of Technology D1 D2 D3 D4 D5 D

2010 SKA Africa Bursary Conference Chalmers University of Technology We can predict the S matrix for the whole array using S parameters in a small part of the array by –Scaling S parameters; –Ignoring mutual coupling between far separated elements.

2010 SKA Africa Bursary Conference Chalmers University of Technology Port definition S 1(3)2(4) port1 port2 dipole3 dipole4

2010 SKA Africa Bursary Conference Chalmers University of Technology 0 0 S=

2010 SKA Africa Bursary Conference Chalmers University of Technology Formula

2010 SKA Africa Bursary Conference Chalmers University of Technology J. Yang and P.-S. Kildal, “Optimizing large log- periodic array by computing a small part of it”, appears in IEEE Trans. on Antennas Propag. Special Issue on Antennas for Next Generation Radio Telescopes, vol. 59, no. 3, March 2011.

2010 SKA Africa Bursary Conference Chalmers University of Technology Reflection coefficient of a 14-element log-periodic Eleven antenna array based on simulation of a 6-element array

2010 SKA Africa Bursary Conference Chalmers University of Technology Six parameters are optimized: –scaling factor k, –dipole length L, –arm width w, –arm spacing da, –transmission line gap dc, –height above ground plane h.

2010 SKA Africa Bursary Conference Chalmers University of Technology GA is used for minimizing S11 in a 6-element folded dipole array. Elite crossover, Roulette wheel selection, crossover and mutation are used in GA. –population size: 50; –5 generations; Simulation tool is CST MS and the optimization is done by in-house Matlab program. Computation Time –Each case 1 hours; –Fully optimized 1 week.

2010 SKA Africa Bursary Conference Chalmers University of Technology 14 pairs of folded dipoles with scaling factor 1.24.

2010 SKA Africa Bursary Conference Chalmers University of Technology The port impedance is 200 Ohms.

2010 SKA Africa Bursary Conference Chalmers University of Technology

2010 SKA Africa Bursary Conference Chalmers University of Technology φ : o with step 1 o. θ : o with step 1 o. Frequency: 2–15 GHz with step 0.1 GHz. Spherical near field measurement

2010 SKA Africa Bursary Conference Chalmers University of Technology

2010 SKA Africa Bursary Conference Chalmers University of Technology

2010 SKA Africa Bursary Conference Chalmers University of Technology Measured Simulated

2010 SKA Africa Bursary Conference Chalmers University of Technology

2010 SKA Africa Bursary Conference Chalmers University of Technology The reflection coefficient is below -10 dB for 2 – 13 GHz. The radiation pattern is constant for 2 – 13 GHz. BOR1 efficiency is –> -0.5 dB for most part of 2 – 13 GHz, –> -1.5 for 2 –13 GHz. Directivity is about 11 dBi. Aperture efficiency is better than – 3 dB for 2 – 13 GHz.

2010 SKA Africa Bursary Conference Chalmers University of Technology